The present invention generally relates to polymeric structures, and more particularly to methods and apparatuses for utilizing electrical properties of a polymer material to monitor a polymer component formed therefrom during the production and/or use of the component.
Polymer components, including seals and the like, are often critical to the performance of the products in which they are used. In such applications, catastrophic failure of the polymer component can be extremely detrimental to the product. Because of this, it is desirable to monitor reductions in performance and structural failures in polymer components. Because polymer components often have complex shapes or are installed in isolated locations, it can be difficult to monitor performance and structural failures with the use of conventional electronic sensors. In particular, the construction and materials for sensing elements of conventional electronic sensors are usually carefully selected to provide a highly responsive, repeatable and specific response to the property (or properties) of interest in order to provide suitable monitoring capabilities, yet the construction and material of a sensing element may not be well suited for use in combination with a polymer component. For example, if attempting to embed or integrate a sensing element into a polymer component, the material selected for the sensing element on the basis of its electrical properties may not be optimal in terms of structural, chemical or other physical properties required for compatibility with the polymer component.
Notwithstanding these challenges, methods and systems have been developed for the purpose of monitoring various polymer materials and structures. For example, U.S. Pat. No. 5,634,497 to Neto, U.S. Pat. No. 6,386,237 to Chevalier et al., and U.S. Pat. No. 6,498,991 to Phelan et al. disclose the detection of a worn hose by sensing the electrical resistivity in one or more wires embedded in the wall of the hose. These patents focus on detecting a discontinuity in the embedded wires, as would result from breakage of the wires due to wear as opposed to sensing a gradual increase in resistivity attributable to wear or deformation of the hose or its wires. U.S. Pat. No. 5,343,738 to Skaggs differs by disclosing a method for capacitively sensing the failure of a hose. In Skaggs, a fuel leakage through an inner layer of a hose is sensed on the basis of the leaked fuel altering the dielectric properties of an insulating material between a pair of copper wires embedded in the hose. Similar to Skaggs, U.S. Pat. No. 5,992,218 to Tryba et al. discloses sensing water leakage through a hose on the basis of the leaked water increasing the conductivity of an electrical insulating layer between a pair of conductor layers separated by the insulating layer. U.S. Pat. No. 5,969,618 to Redmond also discloses a method for detecting the failure of a hose on the basis of electrical conductivity. Redmond's hose is formed to have an annulus containing separated wires, and the failure of the inner layer of the hose is sensed when fluid leaks into the annulus and closes an electric circuit containing the wires.
Another approach to sensing an impending failure of a hose is disclosed in U.S. Pat. No. 4,446,892 to Maxwell. Maxwell discloses a fluid (oil) transport hose formed by at least two plies and a sensing element therebetween. In one embodiment of Maxwell, the sensing element is responsive to the electromagnetic properties of fluid present between the plies as a result of a failure of an inner ply of the hose. In a second embodiment of Maxwell, the sensing element is responsive to the failure of an inner ply of the hose by presenting an open circuit. The sensing element is said to preferably be a coil of fine wire wrapped around the inner ply and connected to means responsive to changes in the electrical impedance (AC) of the coil. Such changes are said to occur from fluid seepage into the material contacting with the coil or deformation of the inner ply, both of which change the inductance of the coil. In an alternative embodiment in which the sensing element is primarily intended to be responsive to the seepage of fluid (oil) between the plies of the hose, Maxwell employs parallel non-touching wires connected to means responsive to a change in conductance between the individual wires or to a change in the capacitance between the wires.
The prior art discussed above is particularly concerned with conduits through which a fluid is conveyed from one location to another, as opposed to fluid vessels such as hydraulic hoses, pipes, and tires in which little if any flow may occur and/or in which structural fatigue of a vessel wall from pressure cycles is often the most important factor in the life of the vessel. Furthermore, sensing systems of the type suggested by Maxwell are generally useful in relatively low pressure systems where the detection of seepage within the hose wall could provide an adequate warning of impending failure. However, in vessels subjected to fluids at relatively high pressures, once seepage occurs catastrophic failure is likely to occur in a matter of seconds, not hours or even minutes. Also, it is possible that a failure could occur in Maxwell's hose without detection as a result of occurring between wires, or a short could occur between wires that does not produce a significant signal.
U.S. Pat. No. 7,555,936 to Deckard addresses the shortcomings of the aforementioned prior art with a method and system capable of sensing and predicting fatigue failures of high-pressure vessel, such as hydraulic hoses or other types of pressurized conduits, as well as other structures subjected to high cyclical pressures. Deckard's system and method utilize a strain-sensing means disposed between an outermost layer of the vessel and an innermost layer of the vessel that is parallel to the outermost layer and contacts the fluid contained by the vessel. The strain-sensing means comprises at least one conductor parallel to the innermost layer of the wall. Changes are sensed in an electrical property associated with the conductor resulting from distortion of the wall of the vessel causing distortion of the at least one conductor. From such changes, a structural failure of the vessel can be predicted well in advance of the failure, allowing the vessel to be safely used for its full life and then replaced before any damage occurs to the fluid system containing the vessel and any objects surrounding the vessel.
U.S. Pat. No. 7,752,904 to Krutz et al. discloses structures having integral life-sensing capabilities as a result of the ability to monitor transitory and permanent distortions of the structures. Such a structure includes a pair of conductive layers and an intermediate layer therebetween formed of a dielectric, semiconductive, or resistive material, such that the conductive and intermediate layers form in combination an electrical element, namely, a capacitive or resistive element. The electrical element is located within the structure so as to be physically responsive to transitory and permanent distortions of the structure resulting from extrinsic and intrinsic sources. An electrical potential is applied to at least one of the conductive layers to generate an electrical signal from the electrical element. Transitory and/or permanent distortions of the structure are monitored by sensing changes in the electrical signal generated by the electrical element in response to the electrical element physically responding to the transitory and/or permanent distortion.